marthastewart

iTrader: (1)

What do you guys thing a good rule of thumb is for timing? The stock tune has a dip right around the middle of the torque curve. I believe the timing should start off low and increase to about 28*. What method do you use to figure out where to start and what number to use? I just got rid of my dip in timing and it is showing 27* all the way through. I dont think that is too good though!

SSpdDmon

iTrader: (10)

The stock timing curve is pretty decent for '99. Improvements in timing aren't going to be really noticeable unless you have a dyno to measure your changes. Get your AFR dialed in and that will provide most of your HP gain in the stock tune.

marthastewart

iTrader: (1)

I read that they were suppose to be good for stock/bolt on cars. But it drops timing in the middle of the torque curve. That does not make sense since its making my timing start off at 28, drop to 21* at 4k rpm and goes back up to 28* at redline.

Most of the logs I looked at showed they did not have the torque drop in the middle. You think I should leave it stock though?

ChrisS@sdpc

Couple of things to consider: Our goal (I assume it is yours as well) is to maximize the power (in reality torque) output of the engine. To a large degree (pun intended), timing is going to be affected by

A) the amount of time allowed for the cycle to complete (i.e. at 6,000 RPM's 1 cycle only lasts 20mS, so we have much less time than we do at say 2,000 RPM's where 1 cycle lasts 3 times longer, 60mS)

B) the load on the engine; by load I am really referring to how hard the engine is working and more importanly how efficiently it is doing the work. When the work is being done really efficiently, maximum VE, there can be less of a need for increased timing. This can be the result of several things like the maximum amount of air is in the cylinder; thus, more particles of air require less advance as it is easier for the flame to propagate. There are of course many factors that will affect at what RPM max VE will occur particularly the valve events (cam shaft design).

There can be other things going on as well that can affect timing because they affect the amount air entering the cylinder like pressure waves or because they affect the efficiency of the mixture/burn like the amount of swirl that is occurring.

You really do need a dyno or something like cylinder pressure equipment to determine the best timing. I would disagree with the above statement that A/F is where power can be found. Even if we limit the discussion to mostly stock vehicles, in my experience different years have vastly different timing tables. Some are aggressive, but many are not. It also depends on the vehicle/application e.g. SUV's and trucks have very low timing tables. Your potential for missing out on maximum power is at much greater risk with the wrong timing versus the wrong A/F. JMO

marthastewart

iTrader: (1)

I got all that but why would a stock tune have the timing at 28* at 2000 rpms then at 4000 rpms lower it to 21 just to bring it back up to 28* at redline. Would it not be better to have it low at the lower rpm then bring it up so your max timing is at red line?

ChrisS@sdpc

Go back and read point 'B'. The result of 'B' can be less timing required. Look at several stock tunes and compare peak VE on the VE table (RPM) to the values in the High Octane Spark table (RPM). I think you will find many of them having lower timing values around peak VE. There are many complicating factors that prevent timing from being a straight line. This is particularly compounded by the fact that most the vehicles aren't single purpose like drag racing. Also, don't assume either that just because the table says 28* that every cylinder wants/needs 28*. Of course with the stock PCM, there isn't much we can do about that.

Also, everything in a stock PCM doesn't always make sense either. The 99 files I have do not go down to 21* though.

SSpdDmon

iTrader: (10)

The stock '99 (A4) file I have shows the following:

2400 ~ 28*
2800 ~ 28*
3200 ~ 29*
3600 ~ 28*
4000 ~ 25*
4400 ~ 24*
4800 ~ 23*
5200 ~ 26*
5600 ~ 28*
6000 ~ 28*
6400 ~ 28*

If you look at the chart, less timing is used during peak torque (peak cylinder pressure) because less timing is needed to light the mixture at the right moment for those conditions. It sounds like you're trying to compare the timing of a PCM-controlled LS1 to that of a traditional SBC with a distributor spinning off of the cam gear. From what I know, they don't quite work the same.

marthastewart

iTrader: (1)

Cool I understand exactly what your talking about. My only tuning experience is like you said with distributor motors and a turbo eclipse. The turbo would start high until the boost hit, it then would drop and from there you would make it advance to max timing at redline.

I am guessing this is nothing like that.

Now with that being said, do most of you see a curve that starts low at low rpm's and goes high, or does everyone with a "GOOD TUNE" have a wave looking timing curve that starts high, goes low at 4800 and back high at redline?

Does anyone have a GOOD HPT file with boltons I could look at that is good. I looked at the bin files and most are with cams and heavy motor work. I usually grasp stuff better when I can see what a "GOOD" example looks like.

SSpdDmon

iTrader: (10)

It depends on your model year. For example, my 2002 started off low, leveled off at ~18* from 4000-4800 and then went up to about 23* at 6000. Based on the C5 tune, I increased the numbers down low so timing hit ~22* before 4000 and kept the drop to 18*. But I could never really run more than 18* from 4000~4800 or more than 23~24* at 6000. The only power mods that car had was a ram air kit, lid, headers and Magnaflow catback.

Now, your '99 is different with an earlier intake, head, and cam design. You can run more timing with your setup and get away with it. What you can try to do is dial in the AFR around 12.5 across the board. Then, get yourself some dyno time and bump up the timing 2*. Make a pass and keep an eye out for KR. If you see some, back off the timing a touch until it goes away. You might be able to increase the timing a little more in some areas than others. But, only keep the changes if they make more power.

Another odd thing is, my '01 H/C SS (M6) uses a '99 table as a base instead of the original spark table. For some reason, it likes 28*~29* at WOT. Go figure...